Currently, a light emitting diode (LED) of a pixel circuit is usually driven by a thin-film transistor (TFT), which may be mentioned as a driving transistor, in a pixel circuit. The driving transistor operates in saturation region to provide a more stable driving current for the LED, since a saturated driving transistor outputs a driving current less sensitive to the source-drain voltage than that output by a driving transistor operating in linear region. FIG. 1 is a schematic diagram of an existing basic pixel circuit. As shown in FIG. 1, the pixel circuit comprises transistors T1 and T12 and a capacitor C11. When the transistor T11 conducts in response to a signal Sn, a data signal data which charges the capacitor C11 and turns on the transistor T1 is input through a node N1. The driving current generated by the transistor T11 drives an LED EL11, between a first power supply ELVDD and a second power supply ELVSS, to emit light. The magnitude of the driving current can be calculated with Equation 1 below.
                              I          EL                =                              1            2                    ⁢          μ          ⁢                                          ⁢                      C            OX                    ⁢                      W            L                    ⁢                                    (                                                V                  GS                                +                                  V                  TH                                            )                        2                                              (                  Equation          ⁢                                          ⁢          1                )            
The μ refers to the mobility of carrier. The Cox refers to the gate oxide capacitance per unit area of the transistor T11. The L refers to the channel length of the transistor T11, and the W refers to the gate width of the transistor T11. The VGS refers to the gate-to-source voltage of the transistor T11, and the VTH refers to the threshold voltage of the transistor T11. However, a threshold voltage drift may cause the threshold voltage of the transistor T11 to be unstable, thus the driving current may drift, leading to the illumination of the LED unstable.
Hence, various circuits, mentioned as threshold compensation circuits, have been developed to eliminate the adverse effect of the threshold voltage drift of the driving transistor. FIG. 2 is a schematic diagram of an existing threshold compensation circuit. Transistors T22 and T23 are turned on in response to the signal Sn to short-circuit a gate electrode and a drain electrode of a driving transistor T21 in a data writing stage. Meanwhile, a transistor T25 is turned off by a signal En, and a transistor T24 is turned off by a signal Sn−1. The data signal data is input by the source electrode of the transistor T21 through the transistor T22. Since the gate and drain electrodes of the transistor T21 are short-circuited, the data signal is input by the gate electrode, charging the capacitor C21, through the drain electrode of the transistor T21. The capacitor continues to be charged until the voltage on the gate electrode of the transistor T22 decreases to (Vdata+VTH) and the transistor T21 is turned off. In a light emitting stage, the transistor T25 is turned on by the signal En, the transistor T24 turned off by the signal Sn−1, and the transistors T22 and T23 turned off by the signal Sn. The power supply ELVDD is provided to the transistor T21 through the transistor T25, while the magnitude of the driving current generated by the driving transistor can be calculated with Equation 2 below.
                              I          EL                =                              1            2                    ⁢          μ          ⁢                                          ⁢                      C            OX                    ⁢                      W            L                    ⁢                                    (                                                V                  ELVDD                                -                                  V                  data                                            )                        2                                              (                  Equation          ⁢                                          ⁢          2                )            
As shown above in Equation 2, the magnitude of the driving current no longer relates to the threshold voltage of the driving transistor T21.
However, an existing threshold compensation circuit, such as the one shown in FIG. 2, has only a single transistor T25 between the power supply ELVDD and the data signal. With the data signal easily affected by the power supply, it is hard to stabilize the light emitting of the LED, since the voltage of the power supply ELVDD is much higher than that of other signals and a leakage current of T25 exists.
Moreover, a pixel circuit, referring to FIG. 2, in existing technology is usually configured to initialize the voltage on the node N2, wherein the voltage mentioned is also the voltage on the gate electrode of the driving transistor T21. Some existing pixel circuits are further configured to initialize the anode of the LED as well. Speaking of a pixel circuit only configured to initialize the gate electrode of the driving transistor, the light emitting of the LED, not initialized, can be unstable. As for a pixel circuit configured to initialize both the gate electrode of the driving transistor and the anode of the LED, the safety of the circuit is insufficient.
To summarize, a situation where the light emitting of an LED is unstable and the safety of the circuit is insufficient exists.